If you need the universal and are picky about the fit it would be best if you can demo the universal version first. My 3DD was very large and hard to wear, and I'm very used to big/protruding IEMs like the tg334 or tralucent 1plus2. You can see the size comparison in my post above. It does looks to me the Mentor is not as large but still seems much larger than asg2.
FYI if I'm to get the 3DD again or the Mentor, I would definitely go for the custom version. Fit is part of the reason but the larger concern is that UM's "universal version" seems just like a demo pair and not really redesigned to sound the same as their custom. For Fitear MH334 and TG334, while using the same drivers, they have totally different housing and sound bores design. Even ASG2 was rearranged and shelled in such a way it would sit flush on your ears.